Thermomechanical properties of amorphous metallic tungsten-oxygen and tungsten-oxide coatings

In this work, we investigate the correlation between morphology, composition, and the mechanical properties of metallic amorphous tungsten-oxygen and amorphous tungsten-oxide films deposited by Pulsed Laser Deposition. This correlation is investigated by the combined use of Brillouin Spectroscopy and the substrate curvature method. The stiffness of the films is strongly affected by both the oxygen content and the mass density. The elastic moduli show a decreasing trend as the mass density decreases and the oxygen-tungsten ratio increases. A plateaux region is detected in correspondence of the transition between metallic and oxide films. The compressive residual stresses, moderate stiffness and high local ductility that characterize compact amorphous tungsten-oxide films make them promising for applications involving thermal or mechanical loads. The coefficient of thermal expansion is quite high (i.e. 8.9 $\cdot$ 10$^{-6}$ K$^{-1}$), being strictly correlated to the amorphous structure and stoichiometry of the films. Under thermal treatments they show a quite low relaxation temperature (i.e. 450 K). They crystallize into the $\gamma$ monoclinic phase of WO$_3$ starting from 670 K, inducing an increase by about 70\% of material stiffness.Comment: The research leading to these results has also received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554). The views and opinions expressed herein do not necessarily reflect those of the European Commissio

Coefficient of thermal expansion of nanostructured tungsten based coatings assessed by thermally induced substrate curvature method

The in plane coefficient of thermal expansion (CTE) and the residual stress of nanostructured W based coatings are extensively investigated. The CTE and the residual stresses are derived by means of an optimized ad-hoc developed experimental setup based on the detection of the substrate curvature by a laser system. The nanostructured coatings are deposited by Pulsed Laser Deposition. Thanks to its versatility, nanocrystalline W metallic coatings, ultra-nano-crystalline pure W and W-Tantalum coatings and amorphous-like W coatings are obtained. The correlation between the nanostructure, the residual stress and the CTE of the coatings are thus elucidated. We find that all the samples show a compressive state of stress that decreases as the structure goes from columnar nanocrystalline to amorphous-like. The CTE of all the coatings is higher than the one of the corresponding bulk W form. In particular, as the grain size shrinks, the CTE increases from 5.1 10$^{-6}$ K$^{-1}$ for nanocrystalline W to 6.6 10$^{-6}$ K$^{-1}$ in the ultra-nano-crystalline region. When dealing with amorphous W, the further increase of the CTE is attributed to a higher porosity degree of the samples. The CTE trend is also investigated as function of materials stiffness. In this case, as W coatings become softer, the easier they thermally expand.Comment: The research leading to these results has also received funding from the European Research Council Consolidator Grant ENSURE (ERC-2014-CoG No. 647554

Laser cleaning of diagnostic mirrors from tokamak-like carbon contaminants

This paper presents a laboratory-scale experimental investigation of laser cleaning of diagnostic First Mirrors (FMs). Redeposition of contaminants sputtered from tokamak first wall onto FMs surface could dramatically decrease their reflectivity in an unacceptable way for the functioning of the plasma diagnostic systems. Laser cleaning is a promising solution to tackle this issue. In this work, pulsed laser deposition was exploited to produce rhodium films functional as FMs and to deposit onto them carbon contaminants with tailored features, resembling those found in tokamaks. The same laser system was also used to perform laser cleaning experiments by means of a sample handling procedure that allows to clean some cm(2) in few minutes. The cleaning effectiveness was evaluated in terms of specular reflectivity recovery and mirror surface integrity. The effect of different laser wavelengths (lambda = 1064, 266 nm) on the cleaning process is also addressed

HOT DUCTILITY OF MICROALLOYED STEELS

The loss in ductility experienced by microalloyed steels at temperatures generally ranging from 700 to 1100°C is a widely studied subject in steel research. The hot ductility behaviour of steels is usually measured through the reduction of area of the samples after hot tensile tests performed up to fracture. The so-called standard hot ductility curves are then obtained by testing the steel at different temperatures. With the aim of achieving more precise information about the hot deformability behaviour of microalloyed steels, interrupted hot-tensile tests followed by rapid quenching (i.e. to “freeze” the microstructure) were carried out. Several steels were characterized by this method, followed by metallurgical investigations of the strained samples in order to identify the damage mechanisms and the precipitates affecting hot ductility. The paper presents a summary of the results achieved about the effects of chemical composition on hot cracking, as affected by precipitation of secondary phases at austenite grain boundarie

Thermal annealing and exposure to divertor-like deuterium plasma of tailored tungsten oxide coatings

In this work we produced tungsten (W) and W oxide (WOx) films by pulsed laser deposition (PLD) with the aim of the addressing modifications of structure and morphology that occur after annealing treatments and high-flux deuterium plasma. Thanks to the high flexibility of PLD we produced nanostructured W containing non-bounded oxygen, different types of WOx and multilayered films. W coatings are dense, non-porous and exhibit a nanocrystalline structure, resembling the coatings used as first wall in tokamaks. The oxide films are nearly stoichiometric amorphous WOx (x = 3) with different morphology from compact to porous. Depending on annealing temperature, nucleation of different crystalline phases (e.g. WO3, W18O49) occurs. Exposure of films to high-flux (similar to 10(24) m(-2) s(-1)) deuterium plasmas in Magnum-PSI at different surface temperatures (T-max = 580 K) determines material modifications at the nanoscale (e.g. nanometric defects) but no delamination. In addition preliminary deuterium retention results are reported

Coordinating Solvent-Assisted Synthesis of Phase-Stable Perovskite Nanocrystals with High Yield Production for Optoelectronic Applications

Inorganic perovskite nanocrystals (NCs) have shown good potential as an emerging semiconducting building block owing to their excellent optoelectronic properties. However, despite extensive studies on their structure-dependent optical properties, they still suffer severely from chemical and phase instabilities in ambient conditions. Here, we report a facile method for the synthesis of mixed halide inorganic perovskite NCs based on recrystallization in an antisolvent mixture in an ambient atmosphere, at room temperature. We introduced an alcohol-derivative solvent, as a secondary antisolvent in the solvent mixture, which crystallizes at room temperature. This mediates and facilitates the perovskite crystallization, leading to a high chemical yield and stability. We demonstrate that this secondary antisolvent establishes intermolecular interactions with lead halide salt, which successfully stabilizes the γ-dark phase of perovskite by encapsulating NCs in a solution and thin film. This allows us to produce concentrated NC solutions with a photoluminescence quantum yield of 70%. Finally, we fabricate CsPbI2Br NCs (optical bandgap 1.88 eV) solar cells, which showed a stabilized photovoltaic performance in ambient conditions, without encapsulation, showing a Voc of 1.32 V

In situ cleaning of diagnostic first mirrors: An experimental comparison between plasma and laser cleaning in ITER-relevant conditions

This paper presents an experimental comparison between the plasma cleaning and the laser cleaning techniques of diagnostic first mirrors (FMs). The re-deposition of contaminants sputtered from a tokamak first wall onto FMs could dramatically decrease their reflectance in an unacceptable way for the proper functioning of plasma diagnostic systems. Therefore, suitable in situ cleaning solutions will be required to recover the FMs reflectance in ITER. Currently, plasma cleaning and laser cleaning are considered the most promising solutions. In this work, a set of ITER-like rhodium mirrors contaminated with materials tailored to reproduce tokamak redeposits is employed to experimentally compare plasma and laser cleaning against different criteria (reflectance recovery, mirror integrity, time requirement). We show that the two techniques present different complementary features that can be exploited for the cleaning of ITER FMs. In particular, plasma cleaning ensures an excellent reflectance recovery in the case of compact contaminants, while laser cleaning is faster, gentler, and more effective in the case of porous contaminant. In addition, we demonstrate the potential benefits of a synergistic solution which combines plasma and laser cleaning to exploit the best features of each technique

A comparison between asymmetric rolling and accumulative roll bonding as means to refine the grain structure of an Al-Mg-Si alloy

The possibility of refining the grain structureof a commercial Al-Mg-Si alloy wasevaluated using asymmetric rolling (ASR) andaccumulative roll bonding (ARB) in the severeplastic deformation (SPD) regime. Bars ofannealed alloy having a thickness of 10 mmwere asymmetrically rolled down to athickness of 0.23 mm with a laboratoryrolling mill featuring the possibility ofindependently modifying the rotational speedof its two rolls. The effect of the rollingtemperature was investigated by tests in therange 150-250°C. A parallel campaign wasalso conducted to investigate the effects ofwarm accumulative roll bonding of the samealloy and in the same temperature range.These tests were carried out on annealedsamples of 1 mm thickness. The experimentalcharacterization (both mechanical andm i c ro s t ructural) demonstrated thatasymmetric rolling and accumulative rollbonding can readily promote the achievementof ultrafine grained structures in Al-Mg-Sialloys